Add the compression parameter to Horovod trainer (#1050)
Add the compression parameter to Horovod trainer (#1050)

* Add the compression parameter to Horovod trainer

* fixed typo

* Change compression default value of HorovodTrainer to None

* Check horovod version

* change import

* fix version check

* fix code for PEP8 requirements

* Update trainers.py

ppwwyyxx/tensorpack,ppwwyyxx/tensorpack